A conventional belt unit of an image forming device is configured of an endless belt, a drive roller that drives the belt to circulate, and a follow roller that rotates following the circulation of the belt. The endless belt is stretched around the drive roller and the follow roller under tension.
In this belt unit, if tension is not applied to the endless belt uniformly in a widthwise direction thereof (i.e., in a direction perpendicular to a rotating direction as well as to a thickness direction of the belt), the belt may, while rotating, skew in a direction from a portion with greater tension toward a portion with smaller tension.
To this effect, in order to prevent the belt from being skewed, belt guides are normally provided on both widthwise ends of the belt such that the drive roller and the follow roller are interposed, respectively in axial directions thereof, between the belt guides.
However, the belt guides demand high quality in both wear resistance and friction coefficient, and are also made from quite expensive materials. Hence, production costs of the belt units tend to inevitably increase.